hilda joao | Goa University, India (original) (raw)

Papers by hilda joao

Marine and Petroleum Geology, May 1, 2022

Marine and Petroleum Geology, Jun 1, 2021

We conducted rock magnetic, mineralogical, sedimentological and geochemical analyses on a sedimen... more We conducted rock magnetic, mineralogical, sedimentological and geochemical analyses on a sediment core (MD161/Stn-11) retrieved from a complex marine sedimentary system of Krishna-Godavari (K-G) basin to delineate the control of mass transport deposits (MTD's) and methane-induced diagenesis on the sediment magnetic record. Four sediment magnetic zones (Z-I, Z-II, Z-III, Z-IV) were defined based on rock magnetic signatures. The sediment magnetic signal is mainly carried by complex magnetic mineral assemblages of detrital (titanomagnetite, titanohematite) and diagenetic (pyrite) minerals. Changes in rock magnetic properties are mainly controlled by fluctuations in supply of detrital magnetic particles, onset of MTD's and differential rate of methane-influenced magnetic minerals diagenesis in the studied sediment core. Downcore reduction in magnetic susceptibility followed by subsequent precipitation of iron sulfides within sediment magnetic zone (Z-I) representing the period of normal sedimentation can be attributed to diagenetic dissolution caused by anaerobic oxidation of methane coupled to sulfate reduction. Decline in magnetic susceptibility and increase in sediment grain size within MTD-rich sediment intervals (Z-II, Z-III, Z-IV) is linked to loss of finer magnetic grains due to diagenetic dissolution and dilution caused by increase in concentration of diamagnetic minerals. Lower values of magnetic grain size diagnostic (ARM/IRM) parameter indicate loss of finer and selective retention of coarser magnetic particles due to diagenetic dissolution beyond 12 mbsf. Elevated content of total organic carbon (TOC) content in Z-III and Z-IV can be attributed to the efficient preservation of labile organic matter due to rapid sediment deposition. A conceptual model is presented to explain the control of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record. Highlights ► Delineated the control of geological and methane-induced diagenetic processes on the sediment magnetic record from the Bay of Bengal. ► Established the linkage between sediment magnetism, mass transport deposits, preservation of organic carbon, sediment gran size, and magnetic mineral diagenesis Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. in a rapidly depositing marine sedimentary system. ► A conceptual model summarizing the control of steady and non-steady sedimentation on the sediment magnetic record is developed.

Geological Magazine, Jan 9, 2023

The oceans store a substantial fraction of carbon as calcium carbonate (CaCO3) and organic carbon... more The oceans store a substantial fraction of carbon as calcium carbonate (CaCO3) and organic carbon (Corg) and constitute a significant component of the global carbon cycle. The Corg and CaCO3 flux depends on productivity and is strongly modulated by the Asian monsoon in the tropics. Anthropogenic activities are likely to influence the monsoon and thus it is imperative to understand its implications on carbon burial in the oceans. We have reconstructed multi-decadal CaCO3 and Corg burial changes and associated processes during the last 4.9 ky, including the Meghalayan Age, from the Gulf of Mannar. The influence of monsoon on carbon burial is reconstructed from the absolute abundance of planktic foraminifera and relative abundance of Globigerina bulloides. Both Corg and CaCO3 increased throughout the Meghalayan Age, except between 3.0–3.5 ka and the last millennium. The increase in Corg burial during the Meghalayan Age was observed throughout the eastern Arabian Sea. The concomitant decrease in the Corg to nitrogen ratio suggests increased contribution of marine organic matter. Although the upwelling was intense until 1.5 ka, the lack of a definite increasing trend suggests that the persistent increase in Corg and CaCO3 during the early Meghalayan Age was mainly driven by higher productivity during the winter season coupled with better preservation in the sediments. Both the intervals (3.0–3.5 ka and the last millennium) of nearly constant carbon burial coincide with a steady sea-level. The low carbon burial during the last millennium is attributed to the weaker-upwelling-induced lower productivity.

The inner shelf off west coast of India, is covered with late Holocene to recent organic rich, si... more The inner shelf off west coast of India, is covered with late Holocene to recent organic rich, silty-clay dominant sediments. High TOC content and sedimentation rates result in high sulfate reduction rates in this region (Mazumdar et al., 2009). In the present study, we have investigated the nature of pyrite and C–Fe–S geo-chemistry in a sediment core (covering the last 378 years) at a water depth of 17 m off Goa, west coast, India, to understand the diagenetic and palaeoclimatic/oceanographic processes. The chromium reducible sulfur (CRS) and highly reactive iron (Fe HR) profiles show significant fluctuations and a negative correlation with δ 34 S CRS , which is attributed to relative availability of highly reactive and less reactive iron bearing minerals close to the sediment water interface. Low δ 34 S CRS corresponding to high CRS content characterizes early diagenetic pyritization near the sediment-water interface, whereas high δ 34 S CRS and low CRS content indicate late diagenetic pyritization of less reactive iron during burial. High OBS/CRS ratios suggest the important role played by the labile organic compound in binding sulfur in the sediment. Partitioning of stable sulfur isotopes into organic and iron bound phases is apparently linked to Fe HR and labile organic matter availability. We have proposed Fe HR profile and Fe HR /Fe T ratios as a potential tool to understand runoff/monsoonal fluctuations in a shallow marine depositional setting. To develop this tool into a potential paleoclimatic/ oceanographic proxy and to link it to other proxies within a chronological framework, needs high resolution sampling and age dating.

Geo-Marine Letters, 2009

We report some main results of multidisciplinary investigations carried out within the framework ... more We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002-2003 in the Krishna-Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW-ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO 4 2− ) concentrations (28.7 to <4 mM), increasing downcore methane (CH 4 ) concentrations (0-20 nM) and relatively high total organic carbon contents (1-2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments. Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gasescape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the Krishna-Godavari Basin offshore.

Sediment cores (~5 m length) from ten stations collected in the water depths of 2665-3210 m in th... more Sediment cores (~5 m length) from ten stations collected in the water depths of 2665-3210 m in the eastern Arabian Sea were studied for pore water sulfate (SO42-), chloride (Cl-) and lighter-hydrocarbons (methane: C1, ethane:C2 and propane: C3) along with other parameters such as calcium carbonate (CaCO3), total organic carbon (TOC) and porosity. Also dissolved oxygen (DO) content in the near bottom waters has been measured. The CaCO3 content of the sediments ranged from 33.6 to 69.9% where as TOC varied from 0.21 to 0.86%. Porosity of the sediments ranged from 54 to 57% while DO of the near bottom waters is in the range of 81 to 150 mM. The sulfate content of the sediments in all the cores varies from 39.8 mM to 12.7 mM indicating SO42- reduction. Similarly chloride concentration varies from 561 to 407 mM. The SO42- / Cl- ratio is in the range of 0.025 to 0.072 with an average ratio of ~ 0.05 indicating that there is no significant loss of sulfate in the pore water. The down-core m...

Marine and Petroleum Geology, 2012

Pore water sulfate consumption in marine sediments is controlled by microbially driven sulfate re... more Pore water sulfate consumption in marine sediments is controlled by microbially driven sulfate reduction via organo-clastic and methane oxidation processes. In this work, we present sediment pore fluid compositions of 10 long sediment cores and high resolution seismic data from the KrishnaeGodavari (KeG) basin, Bay of Bengal. Our results show occurrence of transient (S and kink types) and steady state (quasi-linear) sulfate concentration profiles which are attributed partly to the anaerobic oxidation of methane (d 13 C CH4 : À84.8 to À100.1& VPDB) and organo-clastic sulfate reduction. Influence of AOM on alkalinity is evident from the presence of authigenic carbonate layers with highly depleted carbon isotope ratios in core MD161-8. The authigenic carbonates represent the paleo-SMTZs and suggest marked fluctuation in vertical methane flux. Our geophysical data show the acoustic signatures of upward fluid migration from shallow sub-surface, whereas, coring during NGHP expedition-01 confirms the presence of sub-surface gas hydrate deposits in KeG basin which can be linked to deep methane sources. The geochemical analysis suggests that shallow methane source can be attributed to high burial flux of labile organic matter due to high sedimentation rate. Sampling sites with high methane flux from the shallow gas source are characterized by quasi-linear sulfate concentration profile and a shallow sulfate methane transition zone (SMTZ) and may not be necessarily linked to deeper gas hydrate deposits. In contrast, the deep methane source results in a transient kink type sulfate profile and a deeper SMTZ. We have observed a close link between the occurrence of gas hydrate and the S/kink type sulfate profile. We interpret the short lived 'kink' in the sulfate profiles as a result of recent enhancement in vertical methane flux possibly driven by reactivation of fault-fractures systems which provide the conduits for fluid flow.

Geo-Marine Letters, 2009

We report some main results of multidisciplinary investigations carried out within the framework ... more We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002-2003 in the Krishna-Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW-ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO 4 2− ) concentrations (28.7 to <4 mM), increasing downcore methane (CH 4 ) concentrations (0-20 nM) and relatively high total organic carbon contents (1-2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments. Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gasescape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the Krishna-Godavari Basin offshore.

Geochemistry, Geophysics, Geosystems, 2009

Chemical Geology, 2012

The inner shelf off west coast of India, is covered with late Holocene to recent organic rich, si... more The inner shelf off west coast of India, is covered with late Holocene to recent organic rich, silty-clay dominant sediments. High TOC content and sedimentation rates result in high sulfate reduction rates in this region ). In the present study, we have investigated the nature of pyrite and C-Fe-S geochemistry in a sediment core (covering the last 378 y) at a water depth of 17 m off Goa, west coast, India, to understand the diagenetic and palaeoclimatic/oceanographic processes. The chromium reducible sulfur (CRS) and highly reactive iron (Fe HR ) profiles show significant fluctuations and a negative correlation with δ 34 S CRS , which is attributed to relative availability of highly reactive and less reactive iron bearing minerals close to the sediment water interface. Low δ 34 S CRS corresponding to high CRS content characterizes early diagenetic pyritization near the sediment-water interface, whereas high δ 34 S CRS and low CRS content indicate late diagenetic pyritization of less reactive iron during burial.

AGU Fall Meeting Abstracts, Dec 1, 2008

Recent discovery of gas hydrate and exploration activities in Krishna-Godavari Basin (Bay of Beng... more Recent discovery of gas hydrate and exploration activities in Krishna-Godavari Basin (Bay of Bengal) has generated much interest in geochemistry of sedimentary fluid and diagenetic processes in this region. We are presently investigating the iron-sulfur-carbon coupled system in one of the cores collected on board Marian Dufresne-161 during gas hydrate exploratory cruise by the National Institute of Oceanography in May, 2007. Pore water sulfate concentrations show a'S'shaped non-steady state pattern which may be ...

Abstract: With increased exploitation of the onshore mineral resources, oceans that cover almost ... more Abstract: With increased exploitation of the onshore mineral resources, oceans that cover almost 71% of earth's surface and known as storehouse of minerals, provide a suitable alternative. Amongst the various underwater mineral resources, placer deposits, phosphorites, manganese nodules and encrustations, metalliferous sediments and hydrothermal sulphides are well studied. A variety of mineral deposits are known to occur within the EEZ of India. These include placer deposits along the east and west coast of ...

Current Science, 2006

Abstract: Sediment cores (~ 5 m length) from ten stations collected in the water depths of 2665-3... more Abstract: Sediment cores (~ 5 m length) from ten stations collected in the water depths of 2665-3210 m in the eastern Arabian Sea were studied for pore water sulfate (SO42-), chloride (Cl-) and lighter-hydrocarbons (methane: C1, ethane: C2 and propane: C3) along with other parameters such as calcium carbonate (CaCO3), total organic carbon (TOC) and porosity. Also dissolved oxygen (DO) content in the near bottom waters has been measured. The CaCO3 content of the sediments ranged from 33.6 to 69.9% where as TOC varied ...

Geochemical …, Jan 1, 2007

Several cores from Krishna-Godavari (Western Bay of Bengal) and Goa (Eastern Arabian Sea) basins ... more Several cores from Krishna-Godavari (Western Bay of Bengal) and Goa (Eastern Arabian Sea) basins have been studied for pore water sulfate concentrations in order to understand and contrast diagenetic processes. K-G sediments differ markedly from Goa sediments in pore water sulfate gradients and depth-integrated sulfate reduction rates. Significantly higher sulfate reduction rates in K-G sediments (3.6 to15.8 nmol cm -2 day -1 ), compared to that in Goa sediments (0.011 to 0.65 nmol cm -2 day -1 ), can be attributed to higher sedimentation rates. Lower exposure time to oxygen for the organic matter in K-G sediments has preserved reactive organic compounds required for sulfate reducers and possibly methanogens. Authigenic carbonates with depleted carbon isotopic composition (δ 13 C = -48 to -50) at the base of the sulfate reduction zone in a core from K-G suggests anaerobic methane oxidation (AMO) process. The possibility, that the underlying methane gas hydrate zone is acting as a methane source for sulfate reduction can not be negated specially in light of the existing geophysical studies. Both methane and the fermentation products of organic matter degradation served as a substrate for sulfate reduction. However, a clear understanding of there relative importance needs further investigation.

Marine and Petroleum Geology, May 1, 2022

Marine and Petroleum Geology, Jun 1, 2021

We conducted rock magnetic, mineralogical, sedimentological and geochemical analyses on a sedimen... more We conducted rock magnetic, mineralogical, sedimentological and geochemical analyses on a sediment core (MD161/Stn-11) retrieved from a complex marine sedimentary system of Krishna-Godavari (K-G) basin to delineate the control of mass transport deposits (MTD's) and methane-induced diagenesis on the sediment magnetic record. Four sediment magnetic zones (Z-I, Z-II, Z-III, Z-IV) were defined based on rock magnetic signatures. The sediment magnetic signal is mainly carried by complex magnetic mineral assemblages of detrital (titanomagnetite, titanohematite) and diagenetic (pyrite) minerals. Changes in rock magnetic properties are mainly controlled by fluctuations in supply of detrital magnetic particles, onset of MTD's and differential rate of methane-influenced magnetic minerals diagenesis in the studied sediment core. Downcore reduction in magnetic susceptibility followed by subsequent precipitation of iron sulfides within sediment magnetic zone (Z-I) representing the period of normal sedimentation can be attributed to diagenetic dissolution caused by anaerobic oxidation of methane coupled to sulfate reduction. Decline in magnetic susceptibility and increase in sediment grain size within MTD-rich sediment intervals (Z-II, Z-III, Z-IV) is linked to loss of finer magnetic grains due to diagenetic dissolution and dilution caused by increase in concentration of diamagnetic minerals. Lower values of magnetic grain size diagnostic (ARM/IRM) parameter indicate loss of finer and selective retention of coarser magnetic particles due to diagenetic dissolution beyond 12 mbsf. Elevated content of total organic carbon (TOC) content in Z-III and Z-IV can be attributed to the efficient preservation of labile organic matter due to rapid sediment deposition. A conceptual model is presented to explain the control of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record. Highlights ► Delineated the control of geological and methane-induced diagenetic processes on the sediment magnetic record from the Bay of Bengal. ► Established the linkage between sediment magnetism, mass transport deposits, preservation of organic carbon, sediment gran size, and magnetic mineral diagenesis Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. in a rapidly depositing marine sedimentary system. ► A conceptual model summarizing the control of steady and non-steady sedimentation on the sediment magnetic record is developed.

Geological Magazine, Jan 9, 2023

The oceans store a substantial fraction of carbon as calcium carbonate (CaCO3) and organic carbon... more The oceans store a substantial fraction of carbon as calcium carbonate (CaCO3) and organic carbon (Corg) and constitute a significant component of the global carbon cycle. The Corg and CaCO3 flux depends on productivity and is strongly modulated by the Asian monsoon in the tropics. Anthropogenic activities are likely to influence the monsoon and thus it is imperative to understand its implications on carbon burial in the oceans. We have reconstructed multi-decadal CaCO3 and Corg burial changes and associated processes during the last 4.9 ky, including the Meghalayan Age, from the Gulf of Mannar. The influence of monsoon on carbon burial is reconstructed from the absolute abundance of planktic foraminifera and relative abundance of Globigerina bulloides. Both Corg and CaCO3 increased throughout the Meghalayan Age, except between 3.0–3.5 ka and the last millennium. The increase in Corg burial during the Meghalayan Age was observed throughout the eastern Arabian Sea. The concomitant decrease in the Corg to nitrogen ratio suggests increased contribution of marine organic matter. Although the upwelling was intense until 1.5 ka, the lack of a definite increasing trend suggests that the persistent increase in Corg and CaCO3 during the early Meghalayan Age was mainly driven by higher productivity during the winter season coupled with better preservation in the sediments. Both the intervals (3.0–3.5 ka and the last millennium) of nearly constant carbon burial coincide with a steady sea-level. The low carbon burial during the last millennium is attributed to the weaker-upwelling-induced lower productivity.

The inner shelf off west coast of India, is covered with late Holocene to recent organic rich, si... more The inner shelf off west coast of India, is covered with late Holocene to recent organic rich, silty-clay dominant sediments. High TOC content and sedimentation rates result in high sulfate reduction rates in this region (Mazumdar et al., 2009). In the present study, we have investigated the nature of pyrite and C–Fe–S geo-chemistry in a sediment core (covering the last 378 years) at a water depth of 17 m off Goa, west coast, India, to understand the diagenetic and palaeoclimatic/oceanographic processes. The chromium reducible sulfur (CRS) and highly reactive iron (Fe HR) profiles show significant fluctuations and a negative correlation with δ 34 S CRS , which is attributed to relative availability of highly reactive and less reactive iron bearing minerals close to the sediment water interface. Low δ 34 S CRS corresponding to high CRS content characterizes early diagenetic pyritization near the sediment-water interface, whereas high δ 34 S CRS and low CRS content indicate late diagenetic pyritization of less reactive iron during burial. High OBS/CRS ratios suggest the important role played by the labile organic compound in binding sulfur in the sediment. Partitioning of stable sulfur isotopes into organic and iron bound phases is apparently linked to Fe HR and labile organic matter availability. We have proposed Fe HR profile and Fe HR /Fe T ratios as a potential tool to understand runoff/monsoonal fluctuations in a shallow marine depositional setting. To develop this tool into a potential paleoclimatic/ oceanographic proxy and to link it to other proxies within a chronological framework, needs high resolution sampling and age dating.

Geo-Marine Letters, 2009

We report some main results of multidisciplinary investigations carried out within the framework ... more We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002-2003 in the Krishna-Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW-ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO 4 2− ) concentrations (28.7 to <4 mM), increasing downcore methane (CH 4 ) concentrations (0-20 nM) and relatively high total organic carbon contents (1-2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments. Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gasescape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the Krishna-Godavari Basin offshore.

Sediment cores (~5 m length) from ten stations collected in the water depths of 2665-3210 m in th... more Sediment cores (~5 m length) from ten stations collected in the water depths of 2665-3210 m in the eastern Arabian Sea were studied for pore water sulfate (SO42-), chloride (Cl-) and lighter-hydrocarbons (methane: C1, ethane:C2 and propane: C3) along with other parameters such as calcium carbonate (CaCO3), total organic carbon (TOC) and porosity. Also dissolved oxygen (DO) content in the near bottom waters has been measured. The CaCO3 content of the sediments ranged from 33.6 to 69.9% where as TOC varied from 0.21 to 0.86%. Porosity of the sediments ranged from 54 to 57% while DO of the near bottom waters is in the range of 81 to 150 mM. The sulfate content of the sediments in all the cores varies from 39.8 mM to 12.7 mM indicating SO42- reduction. Similarly chloride concentration varies from 561 to 407 mM. The SO42- / Cl- ratio is in the range of 0.025 to 0.072 with an average ratio of ~ 0.05 indicating that there is no significant loss of sulfate in the pore water. The down-core m...

Marine and Petroleum Geology, 2012

Pore water sulfate consumption in marine sediments is controlled by microbially driven sulfate re... more Pore water sulfate consumption in marine sediments is controlled by microbially driven sulfate reduction via organo-clastic and methane oxidation processes. In this work, we present sediment pore fluid compositions of 10 long sediment cores and high resolution seismic data from the KrishnaeGodavari (KeG) basin, Bay of Bengal. Our results show occurrence of transient (S and kink types) and steady state (quasi-linear) sulfate concentration profiles which are attributed partly to the anaerobic oxidation of methane (d 13 C CH4 : À84.8 to À100.1& VPDB) and organo-clastic sulfate reduction. Influence of AOM on alkalinity is evident from the presence of authigenic carbonate layers with highly depleted carbon isotope ratios in core MD161-8. The authigenic carbonates represent the paleo-SMTZs and suggest marked fluctuation in vertical methane flux. Our geophysical data show the acoustic signatures of upward fluid migration from shallow sub-surface, whereas, coring during NGHP expedition-01 confirms the presence of sub-surface gas hydrate deposits in KeG basin which can be linked to deep methane sources. The geochemical analysis suggests that shallow methane source can be attributed to high burial flux of labile organic matter due to high sedimentation rate. Sampling sites with high methane flux from the shallow gas source are characterized by quasi-linear sulfate concentration profile and a shallow sulfate methane transition zone (SMTZ) and may not be necessarily linked to deeper gas hydrate deposits. In contrast, the deep methane source results in a transient kink type sulfate profile and a deeper SMTZ. We have observed a close link between the occurrence of gas hydrate and the S/kink type sulfate profile. We interpret the short lived 'kink' in the sulfate profiles as a result of recent enhancement in vertical methane flux possibly driven by reactivation of fault-fractures systems which provide the conduits for fluid flow.

Geo-Marine Letters, 2009

We report some main results of multidisciplinary investigations carried out within the framework ... more We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002-2003 in the Krishna-Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW-ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO 4 2− ) concentrations (28.7 to <4 mM), increasing downcore methane (CH 4 ) concentrations (0-20 nM) and relatively high total organic carbon contents (1-2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments. Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gasescape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the Krishna-Godavari Basin offshore.

Geochemistry, Geophysics, Geosystems, 2009

Chemical Geology, 2012

The inner shelf off west coast of India, is covered with late Holocene to recent organic rich, si... more The inner shelf off west coast of India, is covered with late Holocene to recent organic rich, silty-clay dominant sediments. High TOC content and sedimentation rates result in high sulfate reduction rates in this region ). In the present study, we have investigated the nature of pyrite and C-Fe-S geochemistry in a sediment core (covering the last 378 y) at a water depth of 17 m off Goa, west coast, India, to understand the diagenetic and palaeoclimatic/oceanographic processes. The chromium reducible sulfur (CRS) and highly reactive iron (Fe HR ) profiles show significant fluctuations and a negative correlation with δ 34 S CRS , which is attributed to relative availability of highly reactive and less reactive iron bearing minerals close to the sediment water interface. Low δ 34 S CRS corresponding to high CRS content characterizes early diagenetic pyritization near the sediment-water interface, whereas high δ 34 S CRS and low CRS content indicate late diagenetic pyritization of less reactive iron during burial.

AGU Fall Meeting Abstracts, Dec 1, 2008

Recent discovery of gas hydrate and exploration activities in Krishna-Godavari Basin (Bay of Beng... more Recent discovery of gas hydrate and exploration activities in Krishna-Godavari Basin (Bay of Bengal) has generated much interest in geochemistry of sedimentary fluid and diagenetic processes in this region. We are presently investigating the iron-sulfur-carbon coupled system in one of the cores collected on board Marian Dufresne-161 during gas hydrate exploratory cruise by the National Institute of Oceanography in May, 2007. Pore water sulfate concentrations show a'S'shaped non-steady state pattern which may be ...

Abstract: With increased exploitation of the onshore mineral resources, oceans that cover almost ... more Abstract: With increased exploitation of the onshore mineral resources, oceans that cover almost 71% of earth's surface and known as storehouse of minerals, provide a suitable alternative. Amongst the various underwater mineral resources, placer deposits, phosphorites, manganese nodules and encrustations, metalliferous sediments and hydrothermal sulphides are well studied. A variety of mineral deposits are known to occur within the EEZ of India. These include placer deposits along the east and west coast of ...

Current Science, 2006

Abstract: Sediment cores (~ 5 m length) from ten stations collected in the water depths of 2665-3... more Abstract: Sediment cores (~ 5 m length) from ten stations collected in the water depths of 2665-3210 m in the eastern Arabian Sea were studied for pore water sulfate (SO42-), chloride (Cl-) and lighter-hydrocarbons (methane: C1, ethane: C2 and propane: C3) along with other parameters such as calcium carbonate (CaCO3), total organic carbon (TOC) and porosity. Also dissolved oxygen (DO) content in the near bottom waters has been measured. The CaCO3 content of the sediments ranged from 33.6 to 69.9% where as TOC varied ...

Geochemical …, Jan 1, 2007

Several cores from Krishna-Godavari (Western Bay of Bengal) and Goa (Eastern Arabian Sea) basins ... more Several cores from Krishna-Godavari (Western Bay of Bengal) and Goa (Eastern Arabian Sea) basins have been studied for pore water sulfate concentrations in order to understand and contrast diagenetic processes. K-G sediments differ markedly from Goa sediments in pore water sulfate gradients and depth-integrated sulfate reduction rates. Significantly higher sulfate reduction rates in K-G sediments (3.6 to15.8 nmol cm -2 day -1 ), compared to that in Goa sediments (0.011 to 0.65 nmol cm -2 day -1 ), can be attributed to higher sedimentation rates. Lower exposure time to oxygen for the organic matter in K-G sediments has preserved reactive organic compounds required for sulfate reducers and possibly methanogens. Authigenic carbonates with depleted carbon isotopic composition (δ 13 C = -48 to -50) at the base of the sulfate reduction zone in a core from K-G suggests anaerobic methane oxidation (AMO) process. The possibility, that the underlying methane gas hydrate zone is acting as a methane source for sulfate reduction can not be negated specially in light of the existing geophysical studies. Both methane and the fermentation products of organic matter degradation served as a substrate for sulfate reduction. However, a clear understanding of there relative importance needs further investigation.